A V-block measuring apparatus is known in which a workpiece is rotated relative to a V-block having flat sides contacting an external surface of the workpiece at the first and second locations, for example as shown in U.S. patent specification 3274693. This specification also shows in FIGS. 9 and 10 thereof, an apparatus acting in the same way as the V-block apparatus, but for measuring internal surfaces, and having rounded feet for engaging the workpiece at the first and second surfaces.
A disadvantage of the V-block apparatus and of the internally-measuring modification thereto is that the sides of the "V" or the feet follow coarse irregularities in the surface, but not fine irregularities, and therefore the signal from the detector means does not completely represent the surface profile.